Permanent Magnet Moving Coil Instrument PMMC, Overview:
PMMC means permanent magnet moving coil. It is analogue instrument which is used for measuring the current. It is also known as D’Arsonval meter. In this instrument there are two things one is permanent magnet which is an object or material that generates a magnetic field and other is moving coil through which current will flow. Permanent magnet will be used to generate magnetic field the magnetic field generated by this permanent magnet will not visible to the naked eye. It is solely responsible for the unique property of a magnet that attracts ferromagnetic metals such as iron, cobalt and nickel. It either attracts or repels other magnets. A permanent magnet moving coil (PMMC) instrument cannot measure ac current. It can measure only direct current (DC). We can make ammeter and voltmeter from PMCC.
The current that we need to measure is passes through the moving coil as we know that this coil is placed in two permanent magnets so according to Lorentz force law any current carrying conductor which is kept in magnetic field will experience a force and that force can be experience.
Lorentz force= I (L×B)
Construction of PMMC Instrument:
It consists of permanent magnets which are u-shape having two pole one is north Pole and other is south Pole and fixed. The pole face of the magnet is curve. It is made up of hard magnetic material. The magnetic material is made up of ALNICO or ALCOMAX.
Coil is located between two magnets. There will be several turns in the coil. The base of the coil is made up of aluminum which will hold the cupper coil and it will also provide the eddy current damping. Over the aluminum we will wind this coil. There will be cylindrical type mechanism which will be fixed and over it aluminum base will be placed. The purpose of the cylinder is that it will provide radial magnetic field between the two magnets. In order to distribute this magnetic field to this entire coil mechanism we are using this iron cylinder. Also it decreases the reluctance of the air path between the poles and hence increases the flux. Whenever the coil rotates the aluminum base will also rotate. There will be a spindle which will be attached with the moving system. A pointer will be attached with the spring and scale will be kept over the pointer.
The two springs in PMMC are main components of the PMMC. Two helical springs which are in opposite direction are place at the bottom and upper portion of the coil. These springs will control torque. The spring terminal is the leads of the instrument with which we will connect the supply or circuit whose current or voltage we want to measure.
If one of the spring damage then the deflecting torque will become zero due to which the controlling torque of the coil will also be zero. The needle will come to initial position so whenever in PMMC the spring will be damage the pointer value will be zero.
In order to provide free movement to the spindle we will use jewel bearings.
Working Principle of Permanent Magnet Moving Coil Instrument:
When a force will be applied the coil will move so due to which it is called moving coil. When the coil will move the spring S1 and spring S2 will oppose the motion of the coil and it will provide us controlling torque. When the controlling torque will equal to deflecting torque the pointer will be in steady state and here we will take reading from the scale. So this is the basic functionality of permanent moving coil. The pointer is connected with the moving coil. Let us consider a circuit in which one spring is connected with one terminal on the battery while the other spring is connected with other terminal of the battery. The torque will produce when the current will pass through the coil and the pointer will move according to the amount of the current flowing in the circuit. The internal resistance of the PMMC is very small.
The deflecting torque of the PMMC permanent magnet moving coil is proportional to the deflecting torque. Controlling torque is provided by the spring. The damping in PMMC is eddy current damping.
When the current is passes through a coil the deflecting torque is produce. The controlling torque is provided by spring. When the current is pass through the coil force is setup on its both sides which produce deflection torque.
The permanent magnet moving coil PMMC works only for DC voltages. Let suppose we give ac voltage to the PMMC then when positive cycle of the ac voltage occur suppose the coil will move clockwise while when the negative cycle will occur the coil will move anticlockwise. So equivalent torque will occur in every cycle due to which the pointer will not move and only vibrates. In ac voltage at will not give any reading it will only vibrate. In one cycle two torques will be produce one is in clockwise torque while other is anticlockwise torque.
The deflecting torque equation can be represented as:
Td = BINA
B: represent the magnetic field
I: represent the current
N: represent the number of turns in coil
A: represent the area of the cross sectional of the moving coil
The controlling torque is proportional to:
ɵ will be in radians.
- permanent magnet moving coil PMMC will work only in dc circuit it will not work in ac circuits
- The output of the PMMC permanent magnet moving coil will always be linear
- It uses eddy current damping
Errors in PMMC permanent magnet moving coil:
Let suppose provide input current to the permanent magnet moving coil PMMC due which deflection is produce the pointer need to rotate. Now due to fraction the pointer will not move. The scale which is used for measuring the frictional error is torque to weight ratio. When the torque to weight ratio of instrument is high then it means it has low frictional error and when the torque to weight ratio of the instrument is low then it means that it has high frictional error. To increase the torque to weight ratio we will decrease the weight. So to decrease the weight of the pointer it is made from the aluminium.
Due to temperature the internal resistance of the PMMC will change due to which it will produce error. To reduce temperature error we will use swamping resistance which is actually manganin wire whose temperature coefficient is very low.
There will be no frequency error as we are measuring dc voltages.
Advantages of PMMC permanent magnet moving coil:
- permanent magnet moving coil PMMC consume low power
- The scale of the PMMC is uniform
- It has high torque weight ratio
- No hysteresis losses occur in PMMC permanent magnet moving coil
- Range of the PMMC can be increased by using shunts or multipliers
- It has effective and efficient eddy current damping
Disadvantages of PMMC permanent magnet moving coil:
- permanent magnet moving coil PMMC instrument can only be used for the measurement of DC, it cannot be used to measure the AC
- permanent magnet moving coil PMMC cannot be used to measure heavy currents
- Some errors may occur due to aging of control springs and permanent magnets
- Costlier as compare to moving iron instrument
- Friction and temperature may cause error
Moving iron instrument:
These are very simple instruments and can be used in laboratories because they are very cheap and can be manufactured with required accuracy. These are made up of mainly a coil which has several turns. Now we have a piece of iron in front of this coil. When any current flows through this coil it acts like an electromagnetic, therefore it is magnetized and it will attract the piece of iron. So this piece of iron will try to move close towards the electromagnet or solenoid. The iron bar can rotate around this pivot and will attach to a spring.
Types of moving iron instruments:
- Attraction type measuring instrument
- Repulsion type measuring instrument
Attraction type measuring instrument:
It consists of a solenoid and oval shaped soft iron disc. In such a way that it can move in or out of the solenoid and controlling torque is provided by the spring. The damping torque is provided by the air fraction damping. The pointer is attached with iron so that it may deflect along with the moving iron over a graduated scale.
In which a sheet of soft iron is attracted towards solenoid. When a soft iron piece is placed in a magnetic field of current carrying coil it is attracted towards the centre of the coil. Let suppose we have two coils when current is pass through it and magnetic field will be generated. Iron piece is attached with the spindle. This iron will try to go to the centre of the magnetic field. The balance weights will be used with the needle to balance it.
Moving iron instrument Working:
When moving iron instrument is connected to the circuit for measuring the current the operating current flows through stationary coil present in the instrument. A magnetic field is setup and soft iron piece is magnetized which attracted towards the centre of the coil. The pointer attached to the spindle which will deflect over the calibrated scale when the current pass through it. The input can be either ac or dc. When we apply the supply voltage the current will flow in it due which magnetic field will be generated. We will use electromagnet and the coils will be fixed. As we know that every piece of iron is attracting towards the magnetic field. So the iron disc will try to enter the magnetic field. When this iron disc attracted towards the magnetic field the pointer will start deflection and we will receive ɵ. The deflection torque is produce due to the attraction of the iron piece towards the magnetic field in which controlling torque is produced by the springs.
The deflecting torque depends on force acting on iron piece which can be derived by using the following equations:
µ0= Relative permeability
When disc enters to the magnetic field µ0 changes because now the medium for the flux is not air it contain some parts of iron also.
L ⍺ µ0 µr
L=μ0μrN2A / l
When the position of the moving iron changes then the inductance of the coil changes. Torque is differentiation of the energy with respect to the position (ɵ).
In inductor energy stored as:
U = ½ LI2
Td = 1/2 I2 dL/dϴ
M ⍺ H
H ⍺ I
Moving iron instruments Advantages:
- Moving iron instruments are cheap
- These instruments are simple in construction
- As the torque weight ratio is high in moving iron instrument due to which the friction error is very less.
- Accurate at fixed power supply frequency
- Moving iron can be used for measuring direct current and alternating current
Repulsion type measuring instrument:
In which two parallel rods or strips of soft iron magnetized inside a solenoid and are regarded as repelling each other. It consists of two irons in which one is fixed and other is moving. The moving iron is attached with the pointer. Balance weight and control weight is attached with the needle which balance the pointer and remove the unnecessary magnetic field. It consists of two spring one is called upper spring control and other is called lower spring control which control the pointer deflection. To reduce the damping of the pointer we will use air fixing it will reduce the vibration of the needle.
This instrument use electromagnet when the current will pass from the solenoid it will act as electromagnet. As we know that magnet consists of north and south pole. So the coils will act as north and south pole. The moving coil will create opposite poles which is called pole strength. Due to the repulsive force between the fixed iron and moving iron the needle will deflect.
When the repulsive moving iron instrument is connected in the circuit in which we want to measure the current, the operating current flows through the fixed iron and will act as electromagnet. A magnetic field is setup along the axis of the coil and will create electromagnet having north and south poles, this field magnetizes both the pieces fixed iron and moving both the pieces attain similar polarities. A force of repulsion acts between the two, therefore movable piece moves away from the fixed piece.